We have continued our study of the insulin-like growth factor/somatomedin, rat IGF-II, a polypeptide synthesized by a cloned line of rat liver cells (BRL-3A). During the past year, progress has been made in the following areas: (1) determination of the structure of pre-pro-rIGF-II and pro-rIGF-II by molecular cloning and radiosequencing; (2) demonstration that IGF-II mRNA levels are high in fetal and neonatal rat liver and decrease in adult tissues, suggesting that the developmental regulation of IGF-II may occur at the level of transcription; (3) demonstration that the major IGF-II RNA is 4 kilobases, considerably larger than the functional 1.2 kb IGF-II RNA used to establish our cDNA library; (4) demonstration that the 4 kb IGF-II RNA does not direct translation of pre-pro-rIGF-II, and may require further processing to become activated; (5) demonstration that growth hormone regulates the abundance of IGF-I mRNA in adult rat liver, suggesting possible transcriptional regulation; (6) demonstration that the tyrosine kinase activity of the Beta-subunit of the type I IGF receptor closely resembles that of the insulin receptor, suggesting that the homologies between the two receptors extends to their kinase domains; (7) development of antibodies to the Mr 40,000 IGF carrier protein in neonatal rat serum, and demonstrating that it is immunologically distinct from the Mr 150,000 carrier protein in adult rat serum; (8) demonstration that chemically synthesized hybrid molecules containing the A-chain of insulin and the Beta-domain of IGF-I bind with enhanced affinity to IGF carrier proteins; and (9) demonstration using these hybrid molecules that IGF carrier proteins synthesized by cultured human fibroblasts modulate the binding of IGF-I to IGF receptors on cell monolayers.